Ground demineralized cortical and cancellous bone have been widely used in the induction of new bone formation for the treatment of a variety of clinical pathologies. Typically, the bone materials are obtained from human or animal sources, ground and demineralized. Such bone has been demonstrated over the past two decades to induce new bone formation when implanted in animal models, to stimulate elevated levels of the enzyme alkaline phosphatase, and to contain extractable amounts of bioactive molecules, such as bone morphogenetic proteins (BMPs).
The ground demineralized bone matrix (DBM) has also been called demineralized bone (DMB), and demineralized freeze-dried bone allograft (DFDBA). DFDBA materials are provided for clinical use in a freeze-dried state. DBM (or DMB) can be provided for clinical use in either a freeze-dried state or a hydrated state—usually in some form of an aqueous carrier, for example, glycerol in GRAFTON™ (GRAFTON™ is a registered trademark of Osteotech, Inc., Shrewsbury, N.J.), pluronic polymer in DYNAGRAFT™ (DYNAGRAFT™ is a registered trademark of GenSci Regeneration Technologies, Inc., Irvine, Calif.), and collagen in OPTIFORM™ (OPTIFORM™ is a registered trademark of Regeneration Technologies, Inc., Alachua, Fla.). These various commercially available demineralized bone products primarily contain demineralized cortical ground bone distributed for clinical applications. The use of carriers with demineralized bone particles is more acceptable to clinicians because such particles acquire a static charge in the dry state making them difficult to dispense into containers and following rehydration, the clinician typically has difficulties in getting the bone particles to remain at the implant site and in a compacted state wherein they are presumed to be most osteoinductive. DBM is considered to be osteoinductive if it induces the formation of new bone, for example, at the site of clinical application. By adding carriers to the DBM, the biomaterials become easier to aliquot into containers and tend to remain tightly aggregated at the implant site making them easier to handle.
The osteoinductive nature of DBM arises from the interaction between bone-forming cells and the DBM. Such interaction takes place at both a molecular and physical level. At the molecular level, attachment of the bone-forming cells to the DBM involves the presence of “receptors” on the surface of the plasma membrane of mammalian cells that bind to “ligands” present on the surface of the biomaterial. An example of this type of attachment or binding is illustrated in the role of RGD-containing amino acid sequences in the attachment of mammalian cells to a wide variety of molecules present within matrices of tissues. The RGD amino acid sequence refers to the amino acids arginine (R), glycine (G), and aspartic acid (D). Holland, et al. (Biomaterials. 1996. 17(22):2147-56) described the research on a synthetic peptide, gly-arg-gly-asp-ser-pro-lys (GRGDSPK) (which includes the cell-adhesive region of fibronectin, and arg-gly-asp (RGD) peptide sequence covalently bound to a dialdehyde starch (DAS) coating on a polymer surface. The authors concluded that the GRGDSPK/DAS-coated surface could be substituted for an adhesive-protein coated surface in the culture of anchorage-dependent cells.
On the other hand, binding at the physical level in the context of surface patterning has been described, for example, in Goodman, et al. (Biomaterials. 1996. 17(21):2087-95). Goodman et al. described clinical and experimental investigations on manufactured surface topographies that have significant effects on cell adhesion and tissue integration stating that micro- and nano-scale mechanical stresses generated by cell-matrix adhesion have significant effects on cellular phenotypic behavior. Details of surface patterning effects on cell attachment and proliferation were described by Schmidt and Van Recum (Biomaterials. 1992. 13(15):1059-69) measuring macrophage responses to microtextured silicone. Schmidt and Van Recum measured the effects of seven different silicone surface textures on macrophage spreading and metabolic activity in vitro. Variables of the textured arrays important to cell spreading and metabolic activity included size, spacing between, depth, density, and orientation of the individual surface events and the roughness of the surfaces. It was found that pattern dimensions of about 5 micron textures were associated with small cells, whereas a smooth (untextured) surface was associated with large cells. The authors put forth a hypothesis that included a possible mechanism of how a micrometer-sized surface texture could modify cell function.
There are thus several issues pertinent to the ability of implanted bone compositions to induce the formation of bone. These issues include providing an environment suitable for the infiltration of cells, a confined environment that restricts the diffusion of synthesized matrix-forming molecules (for example, collagens, proteoglycans, and hyaluronins), promotes cell attachment to DMBs, and includes the presence of bioactive molecules (for example BMPs). Additionally, the method for making bone fibers for these bone implanted compositions in an efficient and consistent manner is addressed by the present invention.